Method and apparatus for cathodically protecting aluminum alloys against corrosion by alkali nitrate solutions



Apnl 23, 1968 H. B. ROMANS ET AL 3,379,630

METHOD AND APPARATUS FOR CATHODICALLY PROTECTING ALUMINUM ALLOYS AGAINST CORROSION BY ALKALI NITRATE SOLUTIONS Filed May 7, 1965 INVENTORS uz f zw w ATTORNEYS United States Patent of Delaware Filed May 7, 1965, Ser. No. 454,042 6 Claims. (Cl. 204-147) This invention relates to the cathodic protection of aluminum alloy surfaces against corrosive attack by alkali nitrate solutions, More particularly, the invention concerns a novel method and apparatus for the protection of aluminum-magnesium alloy storage vessels against the corrosive action of hot concentrated ammonium nitrate solutions.

Owing to their favorable fabricating properties and mechanical strength, the aluminum-magnesium series of alloys are widely employed as the construction material of choice for both flowing and stagnant or quiescent storage of ammonium nitrate solutions. While the present invention is applicable to the protection of aluminum alloy tanks and storage vessels against corrosion by alkali nitrates in general, it will be illustrated with reference to ammonium nitrate.

Ammonium nitrate is widely employed industrially in the manufacture of nitrogen fertilizers, and of explosives for mining and construction. It is produced by the neutralization of nitric acid with ammonia, followed by concentration of the ammonium nitrate solution to about 83% or higher NH NO by weight. Depending upon the subsequent processing, the 83% solution may be further concentrated and converted to solid form in a prilling tower to condition it for fertilizer use, or it may be crystallized to form solid ammonium nitrate crystals for explosives and general industrial use. In some plants, the concentration range may vary from about 50% to 95%, and the solution temperatures from about 170 to about 200 F. Typically, ammonium nitrate in 83% solution and at a temperature of 180 F. is held in both flowing and stagnant storage in large aluminum-magnesium alloy vessels and tanks, and it is this hot concentrated type of solution which gives rise to most of the corrosion problems. 1

Under laboratory test conditions, aluminum and its alloys exhibit excellent resistance to corrosion attack by ammonium nitrate. However, under actual service con ditions, attack on aluminum-magnesium alloy storage vessels takes place. This is especially true in noninsulated tanks where a large crust of crystallized ammonium nitrate forms on the tank walls. The attack is always localized in nature and the failures occur by pitting. These failures tend to occur more frequently in the tank bottom than on the sidewalls, a condition which may be attributed to a concentration of free nitric acid in the bottom of the tanks.

Study of the pH behavior of ammonium nitrate solutions indicates that concentration cells of free nitric acid are formed, initiated pitting attack forming aluminum nitrate, which in turn can result in pH values as low as 0.5. This highly acid condition is greatly assisted by heat. Thus, if an 83% solution of ammonium nitrate having a pH of 2.7 at 80 F. is heated to 180 F., the pH drops to about 1.0.

Ammonium nitrate storage tankshave been fabricated extensively from high strength 5000 series aluminummagnesium alloys, particularly alloy 5052 (2.5% Mg) and 5454 (2.7% Mg). These alloys are subject to intergranular attack by free nitric acid, owing to the presence of magnesium rich precipitate in the grain boundaries.

It is believed that the formation of small pockets of free nitric acid in a storage tank initiates the intergranular attack, which in turn causes the localized pitting which is the predominant type of attack in the storage tanks. It has been proposed to combat the effect of free nitric acid by keeping the ammonium nitrate solution ammoniacal, but this is costly and requires extra equipment and process steps.

In accordance with the present invention there is provided a novel method for the cathodic protection of aluminum alloy surfaces against the corrosive effects of a concentrated alkali nitrate solution in contact therewith, comprising the steps of making the alloy the cathode, providing an anode immersed in the alkali nitrate solution, and passing direct current from the anode through the solution to the cathode alloy at a current/density sufiicient to liberate alkali metal or ammonium hydroxide adjacent the cathode alloy surface and thereby inhibit attack of the alloy by the solution.

In the case of ammonium nitrate solutions, it has been found that a minimum cathode current density of from about 0.5 to about 1 milliampere per square foot is required to liberate ammonia at the cathode surface. This re sults in a neutralization of free nitric acid present and an upward adjustment of pH level at or near the surface of the exposed aluminum alloy. In the quiescent or stagnant storage of ammonium nitrate solution of from concentration upward, the lower minimum of 0.5 milliampere per sq. ft. is suflicient. In the case of a flowing solution, the minimum will ordinarily be about 1 milliampere. The upper limit of current density values will depend upon solution temperatures and flow rates, but ordinarily about 10 milliamperes will sufiice. The following table shows cathodic current densities and resulting pH values 'at a cathodic aluminum-magnesium 5454 alloy surface, for ammonium nitrate solutions in the concentration range of 50% to 83% NH NO Prior to passage of the current through the solution, the pH of the ammonium nitrate solution, depending upon temperature, may range from about 1.5 to 3.5. As the current is applied the pH rises to about 4, which is equivalent to a neutral point, and thereafter liberation of ammonia raises the pH to non-corrosive levels. The applied voltage will ordinarily be of the order of 2.0 to 3.5 volts.

The accompanying drawing illustrates two arrangements of apparatus suitable for performing the method of the invention, as applied to ammonium nitrate solution storage tanks. In the drawings:

FIGURE 1 is a diagrammatic view, partly in section of a vertical ammonium nitrate storage tank fitted with aluminum anodes and a source of current for cathodic protection;

FIGURE 2 is a diagrammatic view, partly in section of a horizontal tank or tank car body similarly fitted.

Referring to the drawings, FIG. 1 shows a vertical tank 10, which is fabricated of an aluminum-magnesium all-0y, such as 5052 or 5454, provided with a bottom 11 and top 12, and resting on a foundation 13. A body of ammonium nitrate solution 14 fills the tank to the level 15. The system for cathodic protection of the inner surface of the tank comprises an external source of direct current, such as a battery or a rectifier, shown generally at 16. The positive end of the current source is connected to anode system 17, shown in the figure as com- 8,8 3 prising three anodes 18 vertically disposed and spaced apart, and extending downward into the tank via lead-in means 19 to near the bottom 11. The negative end of the current source is connected to the tank wall at a point near the bottom thereof.

In the embodiment shown in FIG. 2, a horizontal storage tank 20, filled with ammonium nitrate solution 21 to level 22 is cathodi-cally protected by a source of current, the positive end of which is connected to an anode 24 via a lead-in 25, while the negative end is connected to the tank shell at point 26.

In accordance with the invention, the anodes are advantageously constructed of aluminum metal, for example of 99.5% purity, which may be made by extrusion, as such or with a stainless steel core. The anode material is preferably about inch in diameter. As shown in the drawings, the anodes are preferably in coiled shape to provide more surface area. They are suspended from the top of the tank and insulated from the tank body with Teflon washers. One or more anodes may be used, appropriately spaced. Preferably the anode area should be about of tank area. Thus, for example, in a tank installation, the tank height was 31 feet, the anodes were 10 in number, located around the circumference of the tank about 10 inches from the wall, and one anode was installed in the center of the tank. The anodes were in coiled shape about 18 inches in diameter, and extended a length of 25 feet.

The operation of the apparatus and the practice of the invention are illustrated by the following example, which is not, however, to be regarded as limiting:

Example 1 A vertical storage tank, of the type described above, made of 5454 alloy and having welds of 5456 alloy was filled with 83% ammonium nitrate solution at a temperature of 180 F. A rectifier was used to furnish power at 3 volts and 17 amperes, with an output of 0.51 watt. The current density maintained on the tank wall when the tank was filled was 1 milliampere per sq. ft., while the current density on the anodes (when new) was 100 millia-mperes per sq. ft. During operation, ammonia was liberated along the tank wall and bottom, While some nitric acid was set free at the anodes. At the end of eight months of use for quiescent storage, no corrosion was noted.

In accordance with another aspect of the invention, the aluminum alloy tank material intended for storing ammonium nitrate solutions of high concentration for extended periods of time can receive additional protection by cladding with high purity aluminum or wrought alloys thereof containing small amounts of alloying elements, such as alloys 1060, 1188, or 7072.

While the presently preferred embodiments of the invention have been illustrated and described, it will be apparent that the invention may be otherwise variously embodied and practiced within the scope of the following claims.

What is claimed is:

'1. Method for the cathodic protection of aluminum 60 prising the steps of making said alloy the cathode, providing an anode immersed in the alkali metal nitrate solution, and passing direct current through the solution at a cathodic current density of at least about 0.5 milliampere per square foot to liberate alkali hydroxide at the cathode alloy surface and thereby inhibit attack on the alloy by said solution.

2. Method for the cathodic protection of aluminum alloy surfaces against the corrosive action of a concentrated ammonium nitrate solution in contact therewith, comprising the steps of making said alloy the cathode, providing an anode immersed in the ammonium nitrate solution, and passing direct current through the solution at a cathodic current density of at least about 0.5 milliampere per square foot to liberate ammonia at the cathode alloy surface and thereby inhibit attack on the alloy by said solution.

3. Method for the cathodic protection of the interior surfaces of aluminum-magnesium alloy storage vessels against the corrosive action of a hot concentrated solution of ammonium nitrate in contact therewith, comprising the steps of making said alloy the cathode, providing at least one aluminum anode extending into the interior of the vessel and immersed in the ammonium nitrate solution, and passing direct current through the solution at a cathodic current density between about 0.5 and about 10 milliamperes per square foot to liberate ammonia at the interior surface and thereby inhibit attack on the alloy by said solution.

4. The method of claim 3 in which the ammonium nitrate solution has a concentration of at least about 50% by weight.

5. A cathodically protected storage vessel for concentrated ammonium nitrate solutions comprising a hollow body of an aluminum-magnesium alloy, a source of direct current located externally of said vessel, means conductively connecting the negative end of said current source to the wall of the vessel, at least one aluminum anode extending downwardly into said vessel so as to be immersed in ammonium nitrate solution contained in said vessel, and means conductively connecting the positive end of said current source to said anode.

6. The apparatus of claim 5 in which the aluminum anode is coil shaped.

References Cited UNITED STATES PATENTS 2,366,796 1/1945 Lawrence et a1 204-147 2,874,105 2/1959 Young 204-148 2,954,332 9/1960 Osborn et al 204148 3,078,993 2/1963 Sheldahl et al. 204-148 3,179,582 4/1965 Preiser 204l97 3,201,335 8/1965 MacNab et a1 204-447 OTHER REFERENCES Pahrney et al., Chemical & Metallurgical Engineering, July 1942, pp. 86 and 87.

HOWARD S. WILLIAMS, Primary Examiner.

T. TUNG, Assistant Examiner. 

1. METHOD FOR THE CATHODIC PROTECTION OF ALUMINUM ALLOY SURFACES AGAINST THE CORROSIVE ACTION OF A CONCENTRATED ALKALI NITRATE SOLUTION IN CONTACT THEREWITH, COMPRISING THE STEPS OF MAKING SAID ALLOY TH CATHODE, PROVIDING AN ANODE IMMERSED IN THE ALKALI METAL NITRATE SOLUTION, AND PASSING DIRECT CURRENT THROUGH THE SOLUTION AT A CATHODIC CURRENT DENSITY OF AT LEAST ABOUT 0.5 MILLIAMPERE PER SQUARE FOOT TO LIBERATE ALKALI HYDROXIDE AT THE CATHODE ALLOY SURFACE AND THEREBY INHIBIT ATTACK ON THE ALLOY BY SAID SOLUTION. 